1. Field of the Invention
The present invention relates to an image blur prevention apparatus for preventing an image blur caused by hand vibration (fluctuation) or the like in a camera, optical equipment, or the like.
2. Related Background Art
In currently available cameras, all operations important to a photographer, e.g., exposure and focus control, are automatically determined. Therefore, even an unskilled user rarely fails in a photographing operation because of automatic determination of exposure or focus control. However, it is difficult to automatically prevent a failure in photographing owing to camera vibration (fluctuation). Recently, a camera capable of preventing a failure in photographing owing to hand vibration of a photographer has been enthusiastically studied. A camera designed to prevent a failure in photographing owing to hand vibration of a photographer, in particular, has been developed and studied.
In general, hand vibration of the photographer using the camera in a photographing operation falls within vibrations (fluctuations) of 1 Hz to 12 Hz. According to a basic concept of taking a picture without any image blur even with hand vibration at the time of a shutter release operation of the camera, camera vibration caused by the above hand vibration is detected, and an image blur is corrected by displacing a correction lens in accordance with the detection value. In order to achieve the above object (i.e., to take a picture without any image blur even with camera vibration), first of all, camera vibration must be accurately detected to correct a change in optical axis due to hand vibration.
In principle, this vibration (camera vibration) can be detected by mounting a vibration sensor and a camera vibration detection system in a camera. The vibration sensor detects an angular displacement, an angular acceleration, an angular velocity, and the like. The camera vibration detection system electrically or mechanically integrates the sensor signal to output an angular displacement. Image blur suppression is then performed by driving a correction optical mechanism for offsetting the photographic optical axis on the basis of the detection information. An image blur suppression system using an angular displacement detection unit will be briefly described below with reference to FIG. 12. This system is designed to suppress an image blur caused by vertical camera vibration (fluctuation) 81p and lateral camera vibration (fluctuation) 81y in directions 81 indicated by the arrows in FIG. 12. Referring to FIG. 12, the system includes a lens barrel 82, and vibration detection system units 83p and 83y for respectively detecting a vertical camera vibration and a lateral camera vibration in angular displacement detecting directions 84p and 84y. The system also includes a correction optical means 85 (coils 86p and 86y for giving a thrust to the correction optical means, and position detection sensors 87p and 87y for detecting the position of the correction optical means). The correction optical means 85 has a position control loop to be described later. The correction optical means 85 is driven in accordance with outputs from the angular displacement detection means 83p and 83y as target values, thereby stabilizing an image surface 88.
When photographing operations were actually performed using this image blur prevention system, it was found that the characteristics of hand vibration in an image blur prevention mode were different from those in a non-image blur prevention mode.
For example, the swing of the body of a person is larger when he/she stands with his/her eyes being closed than when he/she stands with his/her eyes being open. This is because changes in surroundings due to the swing of the body are not input when the person stands with his/her eyes being closed. The image blur prevention mode creates a state similar to the above state in which no changes are input. For this reason, the swing of the body becomes larger than that in the non-image blur prevention mode.
Referring to FIG. 11, the above phenomenon is proved by actual measurement. A two-dimensional PSD 73 (optical position detection means) for measuring an image blur amount is mounted on the image surface of a camera 90. When a user aims the camera 90 at an IRED 74 (projector) as a target, the image blur amount of the image surface of the camera 90 is outputted (an actual measurement 71).
A fluctuation (vibration) detection means (angular velocity meter 84p) on a image blur prevention lens 82 outputs actual hand vibration 72 (a value obtained by integrating the output from the angular velocity meter 84p and converting it into a hand vibration angle).
Referring to FIG. 11, when the image blur prevention system is not operated (see the left half of the drawing), an output 71 coincides with the output 72. As is apparent, this is because the image blur amount of the image surface is equal to the hand vibration angle. When the image blur prevention system is started (see the right half of the drawing, i.e., the right side of "ISON"), the image blur 71 of the image surface (this output is ideally represented by a straight line without any blur) includes fluctuation. Although the image blur amount is considerably reduced by ISON, this image blur reducing effect is not sufficient for a picture without any blur. As described above, this is because the characteristics of hand vibration change in the image blur prevention mode, and large fluctuation having a low frequency appears (75). For this reason, sufficient image blur prevention cannot be performed (fluctuation 76). There are two reasons why large fluctuation having a low frequency cannot be sufficiently prevented. First, the fluctuation detection means cannot accurately detect low frequency fluctuation. In this system, a hand vibration angle is obtained by integrating a hand vibration angular velocity output. In order to accurately integrate a hand vibration band (1 to 12 Hz), the band of the integrator must be broadened to a band of 0.1 to 120 Hz. When the hand vibration band changes to a band of 0.2 to 12 Hz in the image blur prevention mode, the integration band must also be broadened to a band of, e.g., 0.02 to 120 Hz. However, when the integrator is made to respond in a low frequency band of 0.02 Hz, the stability of the integrator (starting characteristics and error output characteristics) is greatly degraded. This method is therefore not practical. For this reason, even if a hand vibration band is broadened to the low frequency side in the image blur prevention mode, the integration band cannot be broadened. As a result, an integration error occurs, and hand vibration cannot be accurately detected. Second, in the image blur prevention mode, the actual hand vibration amount increases, and hence the driving stroke of the correction optical means for performing blur correction becomes too short for correction. As a result, the correction optical means does not accurately follow an output from the fluctuation detection means, leading to degradation in image blur prevention precision. This problem can be solved by increasing the driving stroke of the correction optical means. This, however, poses another problem, i.e., an increase in the size and weight of the image blur prevention system.
As described above, degradation in image blur prevention precision is caused by a change in hand vibration characteristics in the image blur prevention mode. In addition, even if a photographer tries to determine a composition by making a fine framing change in the image blur prevention mode, a change in the posture of the camera, which is made for a framing change, is corrected by the correction optical means. For this reason, a framing change cannot be performed quickly and accurately.